Amorphous Vanadium Phosphate Catalysts Prepared Using Precipitation with Supercritical CO2 as an Antisolvent

Abstract

A new preparative route for vanadium phosphate catalysts is described using supercritical CO2 as an antisolvent. The amorphous microspheroidal VPO produced is shown to be more active than comparable crystalline VPO catalysts for the selective oxidation of n-butane to maleic anhydride and, furthermore, does not require an extensive pre-treatment or activation period to establish full catalytic activity. VPO catalysts prepared using supercritical CO2 as an antisolvent maintain their amorphous nature throughout the catalyst test period. In contrast, amorphous VPO catalysts can also be prepared using liquid CO2 as antisolvent, or by solvent evaporation in vacuo, however, these materials are found to partially crystallise during the oxidation of n-butane. The wholly amorphous catalysts are characterised using transmission electron microscopy, X-ray absorption spectroscopy, 31 P spin echo mapping NMR spectroscopy and X-ray photoelectron spectroscopy. The role of amorphous material in vanadium phosphate catalysis is discussed in detail.

Heterogeneous catalysts—discovery and design

“…Interestingly, and in total contrast to vanadium phosphates prepared using the VPO and VPD methods, 25 the sc-VPO material did not require any reaction time to become active, 31 whereas the VPO and VPD typically require several hours under reaction conditions before they reach their optimal activity. 25,31 Examination of the scCO 2 -VPO material by electron microscopy showed that both the fresh and used catalysts comprised spheroidal particles (Fig. 8) that are not changed on exposure to the reaction gases.…”

“…31,55 The use of scCO 2 as an anti-solvent for the controlled precipitation of materials from conventional solvents has been widely applied to the production of a range of materials. These include polymers, pharmaceutical chemicals, explosives, superconductors, and catalysts.…”

“…31 P spin echo mapping nmr spectroscopy is a powerful technique for the determination of the environment of the vanadium in vanadium phosphates, and typically VOPO 4 phases give a sharp resonance at d ¼ 0 ppm, (VO) 2 P 2 O 7 gives a broad resonance at d ¼ 2400 ppm and disordered V 4+ -V 5+ dimers give a broad signal at d ¼ 1100 ppm. The material derived from VPA-hemihydrate comprises mainly V(V) VOPO 4 phases, the activated VPOhemihydrate shows very little structure in the X-ray diffraction pattern, but by 31 P spin echo mapping nmr spectroscopy the material comprises mainly VOPO 4 phases with some disordered -V 5+ material and a smaller amount of (VO) 2 P 2 O 7 . However, the activated material derived from the VPD-hemihydrate comprises mainly (VO) 2 P 2 O 7 with smaller amounts of disordered V 4+ -V 5+ material and VOPO 4 phases.…”

Active Sites for Propane Oxidation: Some Generic Considerations

“…An adsorbate-type structure of the active site allowing dynamical response will be useful whereas a closely packed 3-D crystalline state will hinder such a process with high activation barriers. Such ''adsorbatelike'' active phases [77,78] can be identified with probe molecule reactions [79][80][81][82][83][84][85][86] and have been rationalized for covering some bulk catalysts [13,42,[87][88][89][90][91] as consequence of surface energy minimization.…”

Synthesis and functional verification of the unsupported active phase of VxOy catalysts for partial oxidation of n-butane

“…For the oxidation of n-butane to maleic anhydride the technical catalyst is vanadium-phosphorusoxide (VPO) [5,6], representing a complex material being composed of a variety of phases [7][8][9]. Some experimental results suggest that in VPO the structure of the catalytically active species is only weakly related to the average bulk structure [10,11].…”